Click a slot to step through empty, want and NNN. The strip under a slot sets its type, and the number says which mod it is: use the same number on both items for the same mod.

Normal: Lands on either base.
NNN: Padding: adds one to the pool, never lands.
Exclusive: Only one can land across the whole result.
Fractured: Dropped from the pool if the other base wins.

How it is calculated

The base is picked 50/50 between the two items.
The number of prefixes and the number of suffixes are each rolled from the pool size, using the table.
Prefixes or suffixes are filled first, 50/50.
Mods are drawn one at a time until the rolled count is reached or no eligible mod is left.

When a side has more than three different wanted mods, the goal is any three of them, since a result holds three. The same number on both items means the same mod. The two share a mod group, so only one copy can land, and wanting it counts once.

Exclusive covers crafted, metacrafted, veiled, essence-only, delve, incursion, aspect and elevated mods. NNN (non-native natural) is padding: it raises the pool size, which raises the chance of more mods, but it cannot transfer to the result. For a wanted mod that only one base can hold, such as an influence mod, leave it Normal and set that item as the base that must win.

The table is the community estimate from the PoE wiki. Every eligible mod is treated as equally likely to be drawn. The game favours rarer mods but the formula is not known, and the 3.26 penalty for heavily padded inputs is not modelled, so treat the results as estimates.